var agent = navigator.userAgent.toLowerCase();

var is_ie = (agent.indexOf("msie") != -1);
var is_ie_4 = (is_ie && (agent.indexOf("msie 4") != -1));
var is_ie_5 = (is_ie && (agent.indexOf("msie 5.0") != -1));
var is_ie_5_up = (is_ie && !is_ie_4);
var is_ie_5_5 = (is_ie && (agent.indexOf("msie 5.5") != -1));
var is_ie_5_5_up = (is_ie && !is_ie_4 && !is_ie_5);

var is_mozilla = ((agent.indexOf("mozilla") != -1) && (agent.indexOf("spoofer") == -1) && (agent.indexOf("compatible") == -1) && (agent.indexOf("opera") == -1) && (agent.indexOf("webtv") == -1) && (agent.indexOf("hotjava") == -1));
var is_mozilla_1_3_up = (is_mozilla && (navigator.productSub > 20030210));